Destroy associated worker tasks and notifications

Summary: Fixes T7107. When destroying an object, destroy associated worker tasks and notifications.

Test Plan:

1. Manually updated the phabricator_worker.worker_activetask table by setting objectPHID to the desired object.
2. Removed object with ./bin/remove destroy.
3. Queried phabricator_worker.worker_activetask table.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7107

Differential Revision: https://secure.phabricator.com/D12822